The analysis of gas mixtures has acquired increasing significance in both process control engineering and monitoring engineering as well as environmental analyses. In addition, the requirements imposed on such measuring systems for gas analysis in terms of measuring sensitivity, long-term stability, selectivity as well as the requirements in terms of the intervals between maintenance procedures and the service life of the measuring systems have been increasing with the increasing degree of automation in industry and environmental monitoring. To recognize gases being discharged in case of defects as quickly as possible, for example, in environmental analyses or in the monitoring of larger industrial plants, it is desirable to cover the areas to be monitored at as close intervals as possible and over as large an area as possible. A large number of sensors, which have locally narrowly limited sensitivity and may be connected with one another via data links, may be used for this.
Far more advantageous and effective are, however, optically imaging gas sensors, in which the light emitted is directed over long measuring sections and wherein the absorption of the light represents the gas species-specific measured effect. Such systems make it possible to obtain data on the mean gas concentration in the measured section and to monitor larger areas.
Such gas detector systems, with a transmitter, with a receiver and with a free measured section located between them, along which the gas composition or the concentration of a certain gas component is detected, are usually called open-path systems, wherein the distance between the transmitter and the receiver, i.e., the length of the free measured section, may be in the range of 200 m. This makes it necessary for the transmitter and the receiver to be exactly aligned with one another in order for the analytical light beam emitted by the transmitter to also actually reach the optical system of the receiver to the full extent. However, even if this is the case, it is, moreover, necessary for the analytical light beam to exactly reach the receiver at the correct site in order to ensure that the highest possible intensity can be detected in the detectors present in the receiver. Consequently, if the transmitter and/or the receiver are not correctly aligned, the system may not operate reliably or with the desired sensitivity.
Such a gas detector system with an open measured section with a laser as the light source and with a receiving element in a common housing is known, for example, from U.S. Pat. No. 5,339,155, in which it is described that light is directed and emitted by means of a semitransparent mirror and an obliquely positioned mirror onto a concave mirror and from there as a parallel beam through the open measured section onto a remote reflector and is reflected from there back to the receiving element into the housing.
An arrangement with an optical gas-measuring system with an open measured section, with a transmitter and with a receiver, is known from U.S. Pat. No. 6,538,728, in which a measuring light source in the transmitter emits an analytical beam into an open measured section, the analytical beam passes through the open measured section, and a detector in the receiver records the analytical beam. A gas concentration of a target gas is determined on the basis of the analytical beam recorded by the detector. Furthermore, two optical or telemetric communication channels, in which a bidirectional data exchange is made possible between the transmitter and the receiver, is disclosed in U.S. Pat. No. 6,538,728. This data exchange makes it possible to obtain information concerning the alignment of the transmitter and receiver at the transmitter and the receiver from the received data by means of data communication and thus to recognize a maladjustment and to support a correction of the adjustment.
Furthermore, arrangements in which an adjusting device with a measuring element 111 is provided for detecting the maladjustment in the alignment, especially tilting of the optical axes 106 of the transmitter 105 and receiver 103 in relation to one another, are known from the state of the art (FIGS. 1a and 1b). The transmitter 105 comprises a light source and a lens assembly 113, and the receiver 103 has a lens assembly 116, a measuring element 111 and a detector 114. Such an arrangement is used by the applicant for gas measurement in open areas, wherein the measuring element 111 is designed as a circular ring, which is arranged at a preset distance from the 114 and on which a number of light-sensitive sensors are arranged at uniformly spaced locations over the entire circumference. This circular ring acts as an apertured diaphragm, which transmits an inner core beam 109 of the parallel light bundle emitted by the light source of the transmitter 105 to the detector 114 and captures an outer marginal beam of the parallel light bundle on the ring itself and on the light-sensitive sensors arranged thereon and thus does not permit it to reach the detector 114. In the adjusted state of the receiver 103, all light-sensitive sensors detect the same intensity. However, some light-sensitive sensors detect no light or little light in case of maladjustment of the receiver 103, and information on the maladjustment can be obtained from this distribution. However, this arrangement of the measuring element 111 leads to an additional loss of light, because part of the light emitted by the transmitter 105, which light is needed for the adjustment of the transmitter and receiver, fails to reach the detector 114 and thus cannot make any further contribution to the measurement at the detector 114.
Moreover, the problem arises, in principle, that the sensitivity is not constant in the detectors being used as a function of the location on the detector surface but increases towards the edge. Consequently, if the beam spot falling on the detector is smaller than the detector surface or the detector surface is not illuminated homogeneously, the measured intensity changes even if there is a shift of the beam spot. However, since the ratio of the intensities measured at the analytical detector and at the reference detector is used as an indicator for the gas concentration, even a slight change in the adjustment of the transmitter or receiver may lead to a significant change in the measured signal.
Another problem is that the transmission range or the cut-off wavelength of the bandpass filters used in open-path systems is a function of the angle of incidence. The change in the respective cut-off wavelengths is especially great if the angle of incidence onto the filter is in the range of 45°. Since such an arrangement is frequently used in gas detector systems to reflect the part that is not transmitted by the filter to another measurement arrangement, incorrect adjustment may soon cause a displacement of the spectral transmission range or the cut-off wavelength here as well, and consequently likewise a changed measurement result.
If the transmitter and receiver are aligned with one another after an approximate adjustment such that the detectors in the receiver “see” the analytical light beam, there still are two possibilities of how the transmitter and receiver are misaligned with one another. On the one hand, the transmitter may be slightly tilted in relation to the connecting line between the transmitter and the receiver, and, on the other hand, it is also conceivable that the receiver is tilted in relation to this connecting line. It is therefore desirable if the transmitter and the receiver are designed such that it is readily possible to identify whether one or both of the above-mentioned cases occur.